1. Field of the Invention
The present invention relates to the electric vehicle drive system, especially to the electric vehicle drive system capable of improving the running performance.
2. Description of the Prior Art
As disclosed in the Official Gazette of Japanese Patent Application Laid-open NO. 133005-1990, the conventional electric vehicle drive system is known to be available in the following types;
a system wherein either the front or rear wheels are driven by two or more motors, a system where each of the front and rear is provided with one motor to form a four-wheel drive, and a system where each of the wheels is driven. Each of them is a drive system comprising a combination of one or more motors of the same type or the same capacity where two or more motors are controlled to produce the same output at all times.
In said technique, the drive efficiency depends on the characteristics of each motor used even when two or more motors are used. For example, when the motor featuring an excellent torque generating efficiency is used in the low speed range, efficient drive capacity cannot be gained in the high speed range even when two or more motors are combined. Conversely, efficiency in the low speed range is reduced by using the motor featuring an excellent torque generating efficiency in the high speed range. This means that drive performances cannot be upgraded over all speed ranges.
Furthermore, the drive system composed of two or more motors of the same type, increases the motor space and curb weight than the drive system using only one motor; this is not advantageous for design configuration of the electric vehicle.
When the drive system is composed of two or more motors so that power is supplied from the battery of the same system, it will be damaged by short-circuiting if any power device of the power converter in said system is damaged. This will cause a short circuit to be formed in the power converter, and short circuit current to flow. So the brake torque is produced until the faulty power device is burnt or the fuse in the circuit is blown and opened.